In recent years, demand for an increase in the size of equipment, such as petrochemical production equipment, storage tanks, and the like, and for large-capacity storage, has caused a steel plate for a pressure vessel used therein to continue to be thickened and, in particular, temperatures, at which toughness may be guaranteed, are gradually decreasing, due to an increase in use of such equipment in cold environments.
Moreover, in manufacturing large-scale structures through processes, such as welding or the like, carbon equivalents (Ceq) have been reduced, and impurities have been controlled to limits able to secure the structural stability of welded portions, as well as of base materials.
In addition, as another method for increasing the internal soundness of welded portions, there is an approach that performs a post weld heat treatment (PWHT) to remove residual stress on welded portions, preventing the deformation of a structure and securing shape and dimensionality.
Generally, PWHTs are conducted on the entirety of structures, but since base materials, in addition to welded portions, are also exposed to heat sources, even if PWHTs proceed locally, the physical properties of the base material may be degraded. As a result, when PWHTs are performed, the quality of base materials is closely related to equipment lifespan.
Meanwhile, when high-strength steel plates for pressure vessels primarily including a hard phase, such as bainite, martensite, martensite-austenite (M/A) constituents, or the like, are subjected to a PWHT for a long period of time, base materials are reduced in strength and are also increased in ductile-brittle transition temperature (DBTT) through a series of processes, such as the rediffusion of carbon, dislocation recovery, grain growth (interface migration of bainite or martensite) and carbide growth, as well as precipitation.
As described above, as a means for preventing a reduction in physical properties caused by a long-term PWHT, there is a first method that increases the amount of added alloying elements capable of increasing hardenability even when Ceqs are high to increase a fraction of a tempered low temperature phase, reducing the extent of a reduction in strength. In addition, there is a second method that increases the content of elements that are effective for solid solution strengthening, such as molybdenum (Mo), copper (Cu), silicon (Si), carbon (C), and the like, in order to increase the matrix strength of ferrite whose structures and dislocation density are not changed after a heat treatment thereof, while realizing the microstructure of quenched-tempered (Q-T) steel as a two-phase structure, including ferrite and bainite, or a three-phase structure, including a certain amount of martensite in addition to the two-phase structure.
However, both of the two methods may be highly likely to degrade the toughness of weld heat affected zones (HAZs) due to an increase in the Ceq, and to raise manufacturing costs owing to the addition of solid solution strengthening elements.
As another method, a precipitation hardening method utilizing rare earth elements is effective under specific composition range and temperature conditions.
Patent Document 1, related to this, discloses that a PWHT time may be guaranteed up to 16 hours by a process of heating and hot rolling a slab including, by wt %, 0.05-0.20% of carbon (C), 0.02-0.5% of silicon (Si), 0.2-2.0% of manganese (Mn), 0.005-0.10% of aluminum (Al), and a balance of iron (Fe) and inevitable impurities, and additionally including one or two or more of copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), niobium (Nb), titanium (Ti), boron (B), calcium (Ca), and rare earth elements, if necessary, of air cooling the slab to room temperature, and of heating the slab at Ac1 to Ac3 transformation points and then slowly cooling it.
However, a PWHT time obtained by the above technology may be very short under the severe conditions of making thick steel plates and of welded portions, and it may be impossible to apply a PWHT for a period of time longer than the obtained PWHT time.
Thus, the development of a steel plate maintaining strength and toughness even after a long-term PWHT under the severe conditions of making thick steel plates and of welded portions, that is, having excellent resistance to a PWHT, is demanded.
Patent Document 1: Japanese Patent Laid-Open No. 1997-256037